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Jet engines work by ingesting oxygen from the air and combusting it with an onboard fuel source like kerosene. But, on planets like Jupiter, with an abundance of hydrogen, would it be possible to remove the onboard fuel source and ingest both oxygen and hydrogen, carrying no fuel at all? This is assuming that the plane would be able to withstand very high pressures, and if Jupiter doesn't have the right amount of oxygen, any other planets that have the right atmosphere to support it.

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    $\begingroup$ "any other planets that have the right atmosphere": There are none in the solar system. Earth with its load of O2 is really an exception, thanks to photosynthesis (GOE). There is no other known place where a significant quantity of 02 can be found. $\endgroup$
    – mins
    Aug 14, 2023 at 17:52

2 Answers 2

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Jupiter has hydrogen, but not oxygen. So you would need to bring your own oxygen, and essentially have just replaced what you use as "fuel."

If the atmosphere was, by itself, flammable then a jet engine should work. For a brief period of time before you set the atmosphere on fire.

Atmospheres don't tend to be flammable, for what are pretty obvious reasons if you think about it.

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    $\begingroup$ @Someone I think that's the point of the first paragraph here. An atmosphere with fuel but no oxygen (Jupiter's) can exist, and your aircraft would have to provide its own O2. An atmosphere with oxygen but no fuel (i.e. Earth's) can exist, and the aircraft brings its fuel. An atmosphere with both fuel and oxygen can exist until the first spark, and then... well, you'll burn up the fuel until it or the oxygen runs out, and then you're back to one of the first two cases. $\endgroup$
    – Ralph J
    Aug 14, 2023 at 16:22
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    $\begingroup$ @manassehkatz-Moving2Codidact Maybe. The possibility that came to mind for me is a very very low density atmosphere wouldn't catch fire under any circumstances simply because there is too much space between the molecules. Perhaps a ramjet could make use of such an atmosphere. $\endgroup$
    – Chris
    Aug 14, 2023 at 18:56
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    $\begingroup$ Maybe at a sufficiently high altitude, you could have such a thin atmosphere that a flame couldn't propagate, but at lower altitude you'd have higher pressure, greater density, and the same first-spark-ignites-the-atmosphere situation. This may be approaching a question better on Worldbuilding.SE than here. $\endgroup$
    – Ralph J
    Aug 14, 2023 at 20:09
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    $\begingroup$ I very much doubt such an atmosphere could exist even at low density--even if it can't support flame you will get reactions sometimes when molecules bump into each other. Thus it would not be stable. $\endgroup$ Aug 15, 2023 at 3:04
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    $\begingroup$ @LorenPechtel: …and yet the atmospheric gas mix doesn't need to be long term stable — it just needs to react slowly enough that other processes (biology, volcanism, photochemistry, etc.) can replenish it. After all, the Earth's atmosphere isn't thermodynamically stable either: it's mostly composed of dinitrogen and dioxygen, which react very slowly to form various nitrogen oxides. But the timescale for this reaction is longer than the biological cycle time for these gases. (Of course that's an endothermic reaction, so you can't really use it to drive an engine even if you catalyzed it.) $\endgroup$ Aug 15, 2023 at 8:22
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To elaborate on the other answer, you don't actually need any fuel or oxidizer to run a jet engine. Jet engines are just a type of open-cycle heat engine where the atmosphere is the working fluid and thrust happens by expelling it as reaction mass. Usually this is implemented as a form of internal combustion engine— Fuel burns inside, that provides the heat, the exhaust shoots out the back, and you go forwards fastly, very speed.

But you can completely remove the fuel-burning step, and replace it with any equivalent source of heat, and it would still be a fully functional jet engine. Nuclear reactors have been tried— No oxygen-burning fuel at all, just a bundle of uranium that the air goes around. Or if you cut a transparent window out of the side of the combustion chamber, and you shone a strong enough laser into it, that could work too.

On Jupiter, you would indeed need to bring your own oxygen/oxidizer, and ingest hydrogen as fuel.

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    $\begingroup$ Or if we're talking science fiction, "burn" the hydrogen with a fusion reaction, no oxygen needed. Only a tiny fraction of the hydrogen has to fuse to heat the rest of it a lot. (But you'd need some kind of cold fusion shenanigans or else that airflow would cool the plasma too much...) $\endgroup$ Aug 16, 2023 at 3:40
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    $\begingroup$ @PeterCordes Hm, indeed. Wouldn't strictly need to be cold fusion shenanigans either; You could localize the fusion to small volumes for very brief durations (E.G. pulsed Z-pinch powered by the turbine), as long as you're not overly insistent on continuous self-sustaining ram-compression-burning. $\endgroup$
    – Will Chen
    Aug 16, 2023 at 4:00
  • $\begingroup$ Yep. Even a battery attached to resistive heating elements would theoretically work. Though finding batteries with high enough capacity that can handle that much power draw will be a challenge (and the battery weight will be far more than carrying a bunch of kerosene or oxygen.) $\endgroup$
    – reirab
    Aug 16, 2023 at 20:42
  • $\begingroup$ that's clever, @PeterCordes !! $\endgroup$
    – Fattie
    Aug 17, 2023 at 11:25
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    $\begingroup$ @PeterCordes Sadly ordinary hydrogen (protium) cannot be fused with protium except by relying on an incredibly rare weak reaction that converts one of the protons into a neutron, which makes a natural hydrogen fusion reactir extremely impractical. Well, I say "sadly" but it keeps our sun from burning all of its fuel extremely fast and going out in a blaze of glory, so perhaps it has its positives as well. ;) $\endgroup$
    – Chris
    Aug 17, 2023 at 15:19

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